A turbocharger is a type of forced induction system used with internal combustion engines. Turbochargers deliver compressed air to an engine intake, allowing more fuel to be combusted, thus boosting the horsepower of the engine without significantly increasing engine weight. As such, turbochargers permit the use of smaller engines that develop the same amount of horsepower as larger, normally aspirated engines. Using a smaller engine in a vehicle has the desired effect of decreasing the mass of the vehicle, increasing performance, and enhancing fuel economy. Moreover, the use of turbochargers permits engine downsizing which results in reduced CO2 emissions, a highly desirable goal for the environment.
Turbochargers typically include a turbine housing connected to the exhaust manifold of the engine, a compressor housing connected to the intake manifold of the engine, and a center bearing housing disposed between and coupling the turbine and compressor housings together. A turbine wheel in the turbine housing is rotatably driven by an inflow of exhaust gas supplied from the exhaust manifold or cylinder head. A shaft is radially supported for rotation in the center bearing housing, and connects the turbine wheel to a compressor impeller in the compressor housing so that rotation of the turbine wheel causes rotation of the compressor impeller. The shaft connecting the turbine wheel and the compressor impeller defines a line which is the axis of rotation. As the compressor impeller rotates, it increases the air mass flow rate, airflow density and air pressure delivered to the cylinders of the engine via the engine intake manifold.
During operation of the turbocharger, the rotating assembly of the turbocharger, comprising turbine wheel, compressor wheel, and connecting shaft, may rotate at 10,000 RPM to 300,000 RPM or more. As part of the turbocharger manufacturing process, the rotating assembly is balanced to reduce vibration, thus improving turbocharger durability. In this regard, the turbine wheel is materially fused to the shaft to make a unitary shaft-and-wheel assembly. The shaft-and-wheel assembly can be accurately machined with shaft diameters ground to tolerances in the 2.5 micron regime; thus, an inherent balance of the shaft-and-wheel assembly is generally within acceptable limits for some applications. In other applications, some component balancing may be required. The compressor wheel, on the other hand, is an extremely difficult part to machine and balance. In addition, the compressor wheel is secured to the free end of the shaft of the shaft-and-wheel assembly via a nut, whereby further imbalance can sometimes be introduced into the rotating assembly.